1. Field of the Invention
The present invention relates to a method of forming a single-crystalline thin film on a base material, and more particularly, it relates to a method for forming a single-crystalline thin film consisting essentially of an oxide superconductor on an arbitrary base material. Throughout the specification, the term "single-crystalline" means a dominative state of a crystal having a specific orientation. This term is applied not only to a single crystal having only a specific orientation but to a crystalline solid containing a mixture of crystals having different orientations with a dominative state of a crystal having a specific orientation.
2. Description of the Background Art
In a technique of fabricating a semiconductor device, various methods such as liquid phase epitaxy (LPE), organometallic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and ion beam epitaxy (IBE) are employed as methods of forming thin films of semiconductor single crystals. These methods, which are capable of forming high quality single-crystalline thin films, are indispensable techniques for fabrication of a semiconductor device.
In the field of superconduction, methods of forming single-crystalline thin films have been studied since discovery of Y, Bi and Tl oxide superconducting materials having critical temperatures of 90 K., 108 K. and 125 K. which are higher than the liquid nitrogen temperature of 77.3 K., in order to apply these materials to electronic devices. It has been found that methods such as laser vapor deposition, reactive vapor deposition and the like are effective for forming high quality single-crystalline thin films with respect to such oxide superconducting materials.
The aforementioned conventional methods of forming single-crystalline thin films utilizing epitaxy, i.e., such a phenomenon that another type of crystal is grown on a specific crystal plane in a constant orientational relation, are generally adapted to form thin films on surfaces of single-crystalline substrates. In each of such conventional methods, it is extremely important to employ a single-crystalline substrate having a crystal structure and a lattice constant which are similar and close to those of the thin film material, in order to form a high quality single-crystalline thin film. In such prior art, therefore, a single-crystalline thin film can be formed only on a substrate which is made of a specific material, while the size of the formable single-crystalline thin film depends on that of the employable substrate. Thus, it is impossible to freely form a single-crystalline thin film having desired size and length according to the prior art.
In the field of a semiconductor thin film, on the other hand, there is graphoepitaxy of employing an amorphous substrate having periodic grooves formed on its surface and generating crystal nuclei on edges of these grooves in a selective orientation, thereby single-crystallizing a film deposited on the substrate. According to this technique, it is possible to form a single-crystalline thin film having excellent crystallinity as to Si, for example, without employing a single-crystalline substrate. Also in such graphoepitaxy, however, the size of the substrate which can be provided with periodic grooves is restricted. Thus, it is difficult to freely form a single-crystalline thin film on a base material having desired size and length, similarly to the above.
In recent years, there has been made an attempt of forming an oxide superconducting film on a flexible long tape base material for manufacturing a superconducting wire. The base material for the wire is generally prepared from a metal, which is a polycrystalline substance in general. When an oxide thin film is formed on such a base material by laser vapor deposition or reactive vapor deposition, generally formed is a polycrystalline or amorphous thin film having random orientation. Even if the thin film has natural orientations, crystals forming the thin film orient specific crystal axes in a direction perpendicular to the surface of the base material, while hardly orienting axes in a direction parallel to the base material surface. Also when an oxide superconductor film is formed on a polycrystalline substrate of MgO, SrTiO.sub.3 or ZrO.sub.2, the as-formed film has irregularly oriented crystal planes. Since a superconducting current is inhibited by grain boundaries, it is impossible to attain sufficient superconductivity in a thin film which is formed on a polycrystalline substrate by the prior art.